Tone control circuit comprising a single potentiometer



y 6, 1970 s. CUTLER 3,514,123

TONE CONTROL CIRCUIT COMPRISING A SINGLE POTENTIOMETER Filed June 23, 1966 2 Sheets-Sheet l 1 200 cgs E ATTENUATIOH OUT M FREQUENCY Ll 22 Q3 4 6" I I2 MQ 26, 1970 s. CUTLER 3,514,123

TONE CONTROL CIRCUIT COMPRISING A SINGLE POTENTIOMETER Filed June 23, 1966 2 Sheets-Sheet 2 FREQUENCY C2oogs l8 T OUT FgeQueNo/ INVENIUR.

QZ ,4 777M United States Patent 3,514,723 TONE CONTROL CIRCUIT COMPRISING A SINGLE POTENTIOMETER Stanley Cutler, Los Angeles, Calif., assignor to Warwick Electronics Inc., Chicago, 111., a corporation of Delaware Filed June 23, 1966, Ser. No. 559,847 Int. Cl. H03h 7/10 US. Cl. 33328 4 Claims ABSTRACT OF THE DISCLOSURE A tone control circuit is described having a single potentiometer for selectively attenuating or boosting either the high or low frequencies of the audio spectrum with respect to a given reference frequency. The circuit comprises a four-terminal LRC network and a common control potentiometer which differentially adjusts the networks transmission characteristics to function as either a low-pass filter, a high-pass filter, or a frequencyflat transmission channel with a continuously variable transition in the gain-versus-frequency response.

This invention relates to tone control circuits and more particularly to circuits for selectively modifying the relative gain of an audio amplifier over a given frequency range.

Various types of tone control circuits have been proposed heretofore, many of which satisfactorily modify the frequency response of an audio amplifier in a desired manner. Typical ones of these prior circuits employ separate controls for boosting or attenuating the so-called bass frequencies, at the lower frequency end of the audible spectrum, and the so-called treble frequencies at the upper end of the spectrum. Additionally, single-control circuits have been proposed which modify tonal response in various ways some of which roll-off the high frequencies in some given manner. Although these prior circuits are suitable for music reproduction and public address systems, they suffer from certain shortcomings which detract from their use in connection with electrical or electronic musical instruments. One shortcoming is that it is difiicult and inconvenient to rapidly manipulate two separate tone controls, such as independent bass and treble controls, which must necessarily interact to some degree with respect to the resulting frequency response curve, while simultaneously playing the instrument. Single tone controls of the prior art have been found to be ineffective in providing the desired range of frequency response adjustment over the entire audio spectrum with particular regard to sharp cut-off in the attenuative hand.

There is provided by the present invention a novel and improved single-control circuit which combines certain advantages of prior multiple-control circuits and which additionally provides certain benefits when used in conjunction with electric or electronic musical instruments. More specifically the invention is particularly useful in connection with bass guitars. The audio frequency spectrum of a note played on a bass guitar, for example, consists of a very low frequency fundamental waveform and a series of higher harmonic waveforms. If, in the process of amplification and reproduction the fundamental tone is emphasized, and the harmonics are attenuated, the resultant sound may be described as round and smooth with a booming quality not unlike a bass drum. If, on the other hand, the fundamental is attenuated and the harmonics are accentuated, the resultant sound will have an entirely different quality which more closely resembles a standard or lead guitar.

3,514,723 Patented May 26, 1970 "ice That is, the harmonic content, rather than the fundamental seems to play the key role in identifying the type of instrument being played.

The circuit of the present invention provides a single control means for selectively attenuating or boosting either the high or low frequencies of the audio spectrum with respect to a given reference or pivot frequency. The circuit to accomplish this comprises a novel fourterminal LRC network which may selectively be adjusted to perform the functions of a low-pass filter, a frequency-flat transmission channel, or a high-pass filter, with a continuously variable transition from one mode to another. The high-pass and low-pass gain-versus-frequency curves are characterized by relatively steep rates of attenuation in the cut-off region.

It is therefore, an object of this invention to provide a novel and improved circuit for differentially controlling the relative bass and treble frequency response of an audio amplifier.

It is another object of this invention to provide a novel and improved single-control audio equalizing circuit which may differentially adjust the frequency response of either the high or the low end of a given pass band.

Yet another object of the inventionis to provide a novel and improved four-terminal LRC network for use with electric or electronic musical instruments.

Other objects and features of the invention will become apparent from the following specification and drawings which show a preferred embodiment of this invention.

In the drawing:

FIG. 1 is a schematic circuit diagram of a preferred embodiment of the invention;

FIG. 2 is an equivalent circuit illustrating the functioning of the apparatus of FIG. 1 in its low-pass filter mode;

FIG. 3 is a graphical plot illustrating the frequency response of the circuit of FIG. 2, wherein relative gain is plotted along the axis of the ordinates and the fre quency is plotted along the axis of the abscissa;

FIG. 4 is an equivalent circuit illustrating the functioning of the apparatus of FIG. 1 in its high-pass filter mode of operation;

FIG. 5 is a graphical plot illustrating the frequency response of the circuit of FIG. 4 wherein the relative gain is plotted along the axis of the ordinate and the frequency is plotted along the axis of the abscissa;

FIG. 6 is an equivalent circuit useful in describing the functioning of the circuit of FIG. 1 when adjusted to its frequency-flat mode of operation;

FIG. 7 is a graphical plot illustrating the frequency response of the circuit of FIG. 6.

Referring to FIG. 1, there is shown a four-terminal LRC network constructed in accordance with the invention. The input signal is applied to a terminal 1 after which it passes through the series combination of a resistor 2 which may be the internal source impedance of the driving stage and an isolating capacitor 3 for blocking any DC from entering the network. The input signal is referenced to a ground terminal 7. The common connection between an inductance 4 and a capacitor 5 is connected to capacitor 3. A voltage-dividing potentiometer 6 comprises the single adjustment or tone-control element of the network. The arm of potentiometer 6 is returned to ground terminal 7. A capacitor 8 is shunt-connected across the arm and one end of potentiometer 6. The frequency response of the circuit may be manually adjusted by appropriately setting the position of the arm of potentiometer 6, as will appear hereinafter. A capacitor 9 and a resistor 11 have a common connection which is tied to shunt a resistor 12 and also an output terminal 13. The output signal is referenced to ground terminal 7.

In a typical application, the above-described network will be inserted in series between amplification stages of an audio frequency signal transmission channel, such channel having sufiicient gain to overcome net insertion loss of the tone-control network when set to its frequencyfiat position.

The operation of the circuit may best be understood by considering equivalent circuits corresponding to the active portions of the principal circuit under given operating conditions. The circuit of FIG. 2 corresponds to the effective portion of the circuit of FIG. 1 if it is assumed that the arm of potentiometer 6 is moved upward (as viewed in FIG. 1) to the junction 14 between capacitors 5 and 9. This condition corresponds to the bass-boost or lowpass filter mode of operation. The components in the network of FIG. 1 having like components in the equivalent circuit of FIG. 2 are identified with similar identifying numbers except that prime marks have been added to the identifying numbers in FIG. 2.

As will be apparent to those versed in the art of filter networks, the equivalent circuit of FIG. 2 comprises three cascaded L-section low-pass filters. The first L-section comprises resistor 2' and capacitor 5', the second section comprises inductance 4' and capacitor 8, and the third cascaded section comprises resistor 11' and capacitor 9'. The shunt resistance of potentiometer 6 in this mode of operation acts to broaden the selectivity of the reactive elements. Resistor 12' is not active as a frequency discriminating element but serves to maintain the equal loudness characteristic of the network. That is, resistor 12' in conjunction with resistor 11 divide the output signal to cause the apparent audible output level in the BASS BOOST position equal to that in the TREBLE BOOST position (to be described more fully hereinafter). Sufiice it to say that resistor 12' is not an essential component if equal-loudness characteristics are not important. As will be appreciated by those versed in the art, the cascading of three separate filter sections will result in a relatively sharp roll-01f of the high frequency.

The approximate frequency response curve of a typical construction of the circuit of FIG. 2 is given in FIG. 3. Classical L-section filter design techniques may be employed to determine the component values for resistors 2 and 11', inductance 4, and capacitors 5', 8, and 9'. Typically the turnover point or 3-decibel point, hereinafter referred to as the pivot frequency, may be set at 200 c.p.s.

There is shown in FIG. 4 the equivalent circuit corresponding to the effective portion of the circuit of FIG. 1 when the arm of the potentiometer 6 is positioned all the way down (as viewed in FIG. 1). Again, like components are similarly identified except for the addition of double primes As will be apparent, this operating mode will result in capacitor 8 being entirely shorted out. The resultant circuit comprises three cascaded L-section high-pass filters serially connected between terminals 1" and 3". The first section comprises resistor 2" and inductance 4". The second high-pass L-section comprises capacitor 5" and resistor 6", and the last cascaded section comprises capacitor 9" and parallel-connected resistors 11" and 12". Cascaded sections give a relatively sharp roll-off of the low frequencies as indicated in the frequency-response curve 15 in FIG. 5.

When the arm of the potentiometer 6 is set approximately at its mid-position, the resulting network will combine elements of both high-pass and low-pass filters, and will therefore give a relatively fiat response characteristic. Specifically, as shown in the equivalent circuit of FIG. 6 (wherein the potentiometer 6 of FIG. 1 is represented by separate resistors 16 and 17) the high-pass filter comprises capacitors 5" and 9", and a resistor 16; the low-pass filter comprises inductance 4", and capacitor 8", and resistor 11". The high-pass and the low-pass filters are connected in parallel with each comprising a separate branch path from the input terminal 1" to the output terminal 13". The combination of these two filters, plus resistors 2" and 12", and series capacitor 3", may be considered as a bridged-T network. The overall response is relatively fiat as indicated by the frequency response curve of FIG. 7.

Ideally, the pivot frequency of the high-pass filter is made to coincide with the pivot frequency of the low-pass filter so that any dip in the central portion of the response curve at the mid-position of the single tone control will be minimized. At all other positions of the tone control 6 intermediate of the end position and/ or the mid-position, there will be corresponding contribution of the high-pass and low-pass filter sections. In effect, the adjusted single control potentiometer operates as a means for selectively and differentially varying the insertion loss of either one or both of the hybrid high-pass and low-pass filters. The relative effectivenes of these two filters also may be varied by appropriate changes in the component values of their respective network branches. Master gain controls may be added to either end of the signal transmission channel in a conventional and well-known manner.

As can be seen from the foregoing description the net audio response curve may be made to pivot about an essentially fixed frequency (typically 200 c.p.s.) to give a virtually infinite number of response curves. In a preferred construction the frequency roll-off on either side of the pivot frequency will be greater than six decibels per octave. The relative position of the pivot frequency will be determined by the actual values chosen for construction of the filter network sections and such selection may follow classical filter-section design methods well known to those versed in the art. The sections thus designed are combined in the novel manner described above, and which incorporates the novel potentiometer arrangement to effect transition through the various modes of operation.

While the invention has been shown and described in terms of a preferred embodiment, it will be readily recognized by those versed in the art that certain omissions, substitutions, modifications and additions may be made without departing from the essential concept of the invention. For example, the number of cascaded filter sections may be augmented or reduced, depending on the desired steepness of the response curves and/ or the overall pass band. Thus, it is intended that the invention be limited only by the scope of the appended claims.

What is claimed is:

1. In an audio signal transmission channel, a network comprising:

an input terminal for receiving an audio signal in said channel;

an output terminal for returning an audio signal to said channel;

a ground return terminal common to said input terminal and said output terminal;

a pair of series capacitances connected between said input terminal and said output terminal;

an inductance and a series resistance connected be tween said input terminal and said output terminal;

a capacitor connected from said ground terminal to the junction between said inductance and said series resistance; and

a single control potentiometer means connected in common with said pair of series capacitances, and said series resistance and inductance for diiferentially controlling the effective insertion loss thereof.

2. The network defined in claim 1 wherein said potentiometer means comprises:

a voltage dividing potentiometer having two end terminals and a movable arm terminal, one of said end terminals being connected to said junction between said series capacitances, the other of said end terminals being connected to said junction between said inductance and said series resistance, and said arm a voltage-dividing potentiometer having a first end lead terminal being connected to said ground terminal. connected to the series connection between said sec- 3. The network defined in claim 1 including: nd and third series-connected capacitors, a second a resistor connected between said ground return termiend lead connected to the series connection between nal and said output terminal. 5 said inductance and said second resistance, and an 4. An adjustable network for series insertion in an arm lead connected to said common reference termi audio signal transmission channel comprising: nal; and

an input terminal for receiving an audio signal in said a fourth capacitor connected between said common channel; reference terminal and the series connection of said an output terminal for returning the audio signal acted inductance and said second resistance.

upon by said network to said channel; a first series signal path between said input and output References Cited terminals comprising a first resistor and first, second, UNITED STATES PATENTS and third series connected capacitors;

a second series signal path extending from the series connection between said first resistor and said second series connected capacitor to said output terminal, comprising an inductance, and a second resistance in 2,505,254 4/1950 Mesner. 2,812,498 11/1957 Hall.

HERMAN :KARL SAALBACH, Primary Examiner series with said inductance; P. L. GENSLER, Assistant Examiner a common reference terminal for said transmission US Cl XR channel;

a third resistance connected between said output terminal and said common reference terminal; 

